This invention relates to a printer for printing characters, symbols, figures, and the like onto a sheet of paper, and more particularly, to a compact printer for providing a color printing.
Typical conventional printers of this type are page printers, which are represented by laser printes.
As illustrated in FIG. 7, according to a page printer, sheets 2 of a given size, such as B5 or A4, which are set in a sheet feeding cassette 1, are fed onto a sheet feeding passage 4 one by one by a hopping roller 3. While the sheet 2 is fed along the sheet feeding passage 4, print data such as characters corresponding to a printing pattern is formed on this sheet by an assembly comprising a photosensitive drum 5. More specifically, the photosensitive drum 5 whose axial length is greater than the maximum width of the sheet 2 is rotated in the direction A as indicated by an arrow in FIG. 7 so that the photosensitive drum 5 is uniformly charged by a charging unit 6 arranged over the peripheral surface of the photosensitive drum 5. Then, an electrostatic latent image corresponding to the printing pattern is formed on the surface of the photosensitive drum 5 by an optical writing unit 7, and toners are selectively supplied to the electrostatic latent image on the photosensitive drum 5 using a developing unit 8, thereby forming a toner image on the photosensitive drum 5. When the toner iamge reaches a transferring unit 9 with a rotation of the photosensitive drum 5, it is transferred onto the sheet 2 by the static attraction caused by the transferring unit 9. The sheet 2 is then separated from the photosensitive drum 5 and is conveyed to a fixing unit 10, where the toner image is fixed onto the sheet 2 by heat and/or pressure. Then, the sheet 2 is discharged onto a discharge tray 11. Meanwhile, after passing the transferring unit 9, the photosensitive drum 5 has its remaining charges removed by a static eliminator 12 and then has its remaining toners cleaned off by a cleaner 13.
FIG. 8 illustrates a page printer which provides a color printing on a sheet 2. Reference numerals used in FIG. 7 are also used in FIG. 8 to denote the corresponding elements. In this color page printer, three separate developing units, 8C, 8M and 8Y, are provided above the peripheral surface of the photosensitive drum 5 to respectively supply cyan, magenta and yellow toners. The transferring unit 9 is disposed in a transferring drum 14, which is designed to wind the sheet 2 around its surface for toner-image transfer and then separate the sheet therefrom.
This color page printer performs the color printing in the following manner. First, a latent image for yellow color is formed on the surface of the photosensitive drum 5 by the optical writing unit 7 in accordance with a printing pattern, and yellow toners are transferred onto this latent image by the developing unit 8Y to form a yellow toner image. The yellow toner image on the surface of the photosensitive drum 5 is transferred, by the transferring unit 9, onto the sheet 2, which has been fed from the sheet feeding cassette 1 and is now wound around the transferring drum 14. The sheet 2 with the yellow toner image transferred thereon rotates on the transferring drum 14 as the transferring drum 14 rotates.
Meanwhile, after passing the transferring unit 9, the charges remaining on the surface of the photosensitive drum 5 are removed by the static eliminator 12 and the yellow toners remaining on the photosensitive drum 5 are cleaned off by the cleaner 13. The photosensitive drum 5 then reaches the charging unit 6 which charges the drums surface again so that the photosensitive drum 5 is ready for formation of the next latent image for magenta color. After the latent image for magenta color is formed on the surface of the photosensitive drum 5 by the optical writing unit 7, magenta toners are transferred onto this latent image by the developing unit 8M, thereby forming a magenta toner image on the surface of the photosensitive drum 5. This magenta toner image is then transferred onto the proper location of the sheet 2 wound around the transferring drum 14, by the transferring unit 9.
This series of processes is also carried out for a toner image for cyan color. When these three colors are mixed in the above manner, printing of the desired color is attained. After the cyan toner image is transferred onto the sheet 2, the sheet 2 is separated from the transferring drum 14, supplied to the fixing unit 10, which fixes the resultant color iamge on the sheet, and is then discharged onto the discharging tray 11.
According to this conventional page printer, however, the photosensitive drum 5 is designed to have its axial length greater than the width of the sheet 2. This necessitates that the axial length of each of various units arranged around the photosensitive drum 5, such as the charging unit 6, the optical writing unit 7, developing units 8, 8C, 8M and 8Y, the transferring unit 9, the static eliminator 12, the cleaner 13 and the transferring drum 14, should be nearly equal to the axial length of the photosensitive drum 5. This enlarges the overall size of the page printer and increases the manufacturing cost as a consequence.
The assignee of this application has already proposed a printer to solve the above problem. In this improved printer, the photosensitive drum 5 having a longer axial length than the width of the sheet 2 is replaced with a latent image supporter with an endless belt shape which runs in the width direction of the sheet 2. This design improvement can reduce the size of the overall printer as well as the manufacturing cost and can also provide a high quality printing.